A typical automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No. 3,844,820 Burgett et al. It has a zirconia sensing element shaped as a tapered thimble. One end is opened and has a thick circumferential flange. The other end is disclosed and forms the most active part of the element. The interior and exterior of the thimble has separate porous inner electrode coatings of platinum or the like.
The inner electrode is exposed to a known source of oxygen, such as air or a mixed metal oxide, for establishing a reference potential. This electrode is generally formed by painting a coating of platinum ink into the zirconia thimble, drying the coating, then firing the coated thimble at an elevated temperature. The composition and dimensions for such a thimble, and an improved technique for applying the reference electrode, is disclosed in U.S. Pat. No. 4,264,647 entitled "Reference Electrode Printing Process and Mask for Exhaust Gas Oxygen Sensor", which was filed on Oct. 1, 1979 in the name of John Trevorrow.
The outer electrode is usually formed by thin film techniques, such as evaporation or sputtering. Improved sputtering techniques for applying the outer, i.e. exhaust, electrode are disclosed in U.S. Pat. No. 4,253,931 Gold et al; and U.S. Ser. No. 189,732 Gold et al.
U.S. Pat. No. 4,253,931 describes an improved sputtering process which involves the use of nitrogen and/or oxygen along with argon as a pressure of about 10-20 millitorr to provide higher yields of fast responding sensors as formed. It also discloses a wide target thimble spacing of about 3.8 cm along with a high sputtering of 13-22 watts/cm.sup.2 of target area. The foregoing improved sputtering techniques all help produce sensors having fast average sensor response times without artificial ageing. However, it must be recognized that the zirconia thimbles were coated on a batch basis, and that not all thimbles in a given sputtering batch will exhibit the same fast response time when assembled into finished sensors. U.S. Pat. No. 4,253,934 Berg et al describes salvaging the thimbles producing slower response times by nitrogen ageing them. This involves treating them in substantially oxygen-free nitrogen at an elevated temperature. However, there is no practical way to discern a fast responding thimble from a low responding thimble, before assembly into a finished sensor. Thus, an effective way to insure highest yield of fast responding sensors, is to nitrogen age all of the thimbles before assembly into sensors. The increase in yield has been sufficient to offset the costs of such a treatment.
I have now found a further improvement in the aforementioned sputtering techniques, which can provide a still further improvement in yield of fast responding sensors. The yield increase appears to be so significant that it may even obviate the significant benefits attributable to the nitrogen ageing treatment of the aforementioned U.S. Pat. No. 4,253,934 Berg et al. Tests have shown a yield increase that makes the additional nitrogen ageing treatment of questionable economic value. In the very least, my improved process should provide an improvement in yield even after the nitrogen artificial ageing of the Berg et al U.S. Pat. No. 4,253,934. In other words, one can consider my invention provides a faster average sensor response time.